1. Field of the Invention
The present invention relates to a method for the ultrasonic detection of cold weld defects occurring at the weld zone in seam welded pipe.
2. Description of the Prior Art
In general, the conventional ultrasonic flaw detection method used for electric welded steel pipe has been the circumferential angle beam technique illustrated in FIG. 1 of the accompanying drawings. In FIG. 1, an ultrasonic beam 5 transmitted from a probe 3 is projected through water 4 into a pipe 1 at an angle of incidence i and is refracted at an angle .theta.. Any defect encountered in the pipe by the ultrasonic beam will reflect the beam back along the same path to be received by the probe 3.
The frequency f of the signal from the probe in the above circumferential angle beam technique usually has been set at 2.25 MHz to 5 MHz and a refractive angle .theta. of 37.degree. to 90.degree. has been used for detecting a flaw in the weld zone 2 of the pipe. In the case of steel pipe, if the angle of incidence i is set at 16.0.degree. to 27.3.degree., the refractive angle .theta. will fall within the above mentioned range. Further, in order to attain certain detection of flaws in the weld zone 2, electromagnetic induction systems, optical systems, paint mark systems, magnetic mark systems, and visual systems, and the like are used to ensure that the probe 3 follows the weld zone 2 during the detection. Furthermore, plural numbers of probes 3 are positioned with a set interval between each probe for maintaining the accuracy of the detection and tracking.
In ultrasonic flaw inspection in accordance with the conventional methods, penetrators existing in the weld zone 2 can be detected with certainty. Moreover, as shown in FIG. 2, because not only the weld zone 2 but also the parent metal in the proximity of the weld zone are subjected to the ultrasonic beam, the penetrator C in the weld zone 2 together with inclusions A and B, which cause little harm, can be detected. However, cold weld defects, existing in the weld zone of the electric welded steel pipe are not detected.
The cold weld defects which are produced when the welding heat input is low, are very fine oxides no larger than several microns, and consist mainly of FeO. Those oxides form an aggregation at the weld zone. The presence of a cold weld defect in the weld zone results in considerable reduction in the toughness of the pipe. It is known that the above cold weld defects are undetectable using conventional ultrasonic flaw inspection methods of the prior art or any other non-destructive testing method (W. Stumm: Magnetic stray-flux measurement for testing welded tubes on line, Non-destructive Testing, Vol. 9, No. 1 (1976), Pages 3-8).
Accordingly, the presence of cold weld defects has been discerned by other inspection methods, such as carrying out a rupture test via by fatigue test or a Charpy impact test conducted on samples, and then by observing the face of the fracture with a microscope.